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<span class="pre noprint docinfo">[<a href="https://www.rfc-editor.org" title="RFC Editor">RFC Home</a>] [<a href="/rfc/rfc7941.txt">TEXT</a>|<a href="/rfc/pdfrfc/rfc7941.txt.pdf">PDF</a>|<a href="/rfc/rfc7941.html">HTML</a>] [<a href='https://datatracker.ietf.org/doc/rfc7941' title='IETF Datatracker information for this document'>Tracker</a>] [<a href="https://datatracker.ietf.org/ipr/search/?rfc=7941&amp;submit=rfc" title="IPR disclosures related to this document">IPR</a>] [<a href='https://www.rfc-editor.org/info/rfc7941' title='Info page'>Info page</a>]                  </span><br/><span class="pre noprint docinfo">                                                                        </span><br /><span class="pre noprint docinfo">                                                       PROPOSED STANDARD</span><br /><span class="pre noprint docinfo">Updated by: <a href="/rfc/rfc8843" target="_blank">8843</a>, <a href="/rfc/rfc9143" target="_blank">9143</a>                                                  </span><pre>Internet Engineering Task Force (IETF)                     M. Westerlund
Request for Comments: 7941                                     B. Burman
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                                  R. Even
                                                     Huawei Technologies
                                                               M. Zanaty
                                                           Cisco Systems
                                                             August 2016


                        <span class="h1">RTP Header Extension for</span>
        <span class="h1">the RTP Control Protocol (RTCP) Source Description Items</span>

Abstract

   Source Description (SDES) items are normally transported in the RTP
   Control Protocol (RTCP).  In some cases, it can be beneficial to
   speed up the delivery of these items.  The main case is when a new
   synchronization source (SSRC) joins an RTP session and the receivers
   need this source's identity, relation to other sources, or its
   synchronization context, all of which may be fully or partially
   identified using SDES items.  To enable this optimization, this
   document specifies a new RTP header extension that can carry SDES
   items.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7941">http://www.rfc-editor.org/info/rfc7941</a>.













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<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
     <a href="#section-2.1">2.1</a>.  Requirements Language . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
     <a href="#section-2.2">2.2</a>.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
   <a href="#section-3">3</a>.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
   <a href="#section-4">4</a>.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-5">5</a>
     <a href="#section-4.1">4.1</a>.  SDES Item Header Extension  . . . . . . . . . . . . . . .   <a href="#page-5">5</a>
       <a href="#section-4.1.1">4.1.1</a>.  One-Byte Format . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
       <a href="#section-4.1.2">4.1.2</a>.  Two-Byte Format . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
     <a href="#section-4.2">4.2</a>.  Usage of the SDES Item Header Extension . . . . . . . . .   <a href="#page-6">6</a>
       <a href="#section-4.2.1">4.2.1</a>.  One-Byte or Two-Byte Headers  . . . . . . . . . . . .   <a href="#page-6">6</a>
       <a href="#section-4.2.2">4.2.2</a>.  MTU and Packet Expansion  . . . . . . . . . . . . . .   <a href="#page-7">7</a>
       <a href="#section-4.2.3">4.2.3</a>.  Transmission Considerations . . . . . . . . . . . . .   <a href="#page-8">8</a>
       <a href="#section-4.2.4">4.2.4</a>.  Different Usages  . . . . . . . . . . . . . . . . . .   <a href="#page-9">9</a>
       <a href="#section-4.2.5">4.2.5</a>.  SDES Items in RTCP  . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
       <a href="#section-4.2.6">4.2.6</a>.  Update Flaps  . . . . . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
       <a href="#section-4.2.7">4.2.7</a>.  RTP Header Compression  . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-5">5</a>.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-5.1">5.1</a>.  Registration of an SDES Base URN  . . . . . . . . . . . .  <a href="#page-11">11</a>
     5.2.  Creation of the "RTP SDES Compact Header Extensions"
           Sub-Registry  . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
     <a href="#section-5.3">5.3</a>.  Registration of SDES Item . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-6">6</a>.  Security Considerations . . . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#section-7">7</a>.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
     <a href="#section-7.1">7.1</a>.  Normative References  . . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
     <a href="#section-7.2">7.2</a>.  Informative References  . . . . . . . . . . . . . . . . .  <a href="#page-14">14</a>
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-17">17</a>






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<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This specification defines an RTP header extension [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>][RFC5285]
   that can carry RTCP Source Description (SDES) items.  Normally, the
   SDES items are carried in their own RTCP packet type [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].  By
   including selected SDES items in a header extension, the
   determination of relationship and synchronization context for new RTP
   streams (SSRCs) in an RTP session can be optimized.  Which
   relationship and what information depends on the SDES items carried.
   This becomes a complement to using only RTCP for SDES item delivery.

   It is important to note that not all SDES items are appropriate to
   transmit using RTP header extensions.  Some SDES items perform
   binding or identify synchronization contexts with strict timeliness
   requirements, while many other SDES items do not have such
   requirements.  In addition, security and privacy concerns for the
   SDES item information need to be considered.  For example, the Name
   and Location SDES items are highly sensitive from a privacy
   perspective and should not be transported over the network without
   strong security.  No use case has identified that such information is
   required when the first RTP packets arrive.  A delay of a few seconds
   before such information is available to the receiver appears
   acceptable.  Therefore, only appropriate SDES items, such as CNAME,
   will be registered for use with this header extension.

   Requirements language and terminology are defined in <a href="#section-2">Section 2</a>.
   <a href="#section-3">Section 3</a> describes why this header extension is sometimes required
   or at least provides a significant improvement compared to waiting
   for regular RTCP packet transmissions of the information.  <a href="#section-4">Section 4</a>
   provides a specification of the header extension and usage
   recommendations.  <a href="#section-5">Section 5</a> defines a subspace of the header
   extension URN to be used for existing and future SDES items and
   registers the appropriate existing SDES items.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Definitions</span>

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  Requirements Language</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].










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<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Terminology</span>

   This document uses terminology defined in "A Taxonomy of Semantics
   and Mechanisms for Real-Time Transport Protocol (RTP) Sources"
   [<a href="./rfc7656" title="&quot;A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources&quot;">RFC7656</a>].  In particular, the following terms are used:

      Media Source

      RTP Stream

      Media Encoder

      Participant

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Motivation</span>

   SDES items are associated with a particular SSRC and thus with a
   particular RTP stream.  The Source Description items provide various
   metadata associated with the SSRC.  How important it is to have this
   data no later than when the first RTP packets is received depends on
   the item itself.  The CNAME item is one item that is commonly needed
   either at reception of the first RTP packet for this SSRC or at least
   by the time the first media can be played out.  If it is not
   available, the synchronization context cannot be determined; thus,
   any related streams cannot be correctly synchronized.  Therefore,
   this is a valuable example for having this information early when a
   new RTP stream is received.

   The main reason for new SSRCs in an RTP session is when media sources
   are added.  This can be because either an endpoint is adding a new
   actual media source or additional participants in a multi-party
   session are added to the session.  Another reason for a new SSRC can
   be an SSRC collision that forces both colliding parties to select new
   SSRCs.

   For the case of rapid media synchronization, one may use the RTP
   header extension for rapid synchronization of RTP flows [<a href="./rfc6051" title="&quot;Rapid Synchronisation of RTP Flows&quot;">RFC6051</a>].
   This header extension carries the clock information present in the
   RTCP sender report (SR) packets.  However, it assumes that the CNAME
   binding is known, which can be provided via signaling [<a href="./rfc5576" title="&quot;Source-Specific Media Attributes in the Session Description Protocol (SDP)&quot;">RFC5576</a>] in
   some cases, but not all.  Thus, an RTP header extension for carrying
   SDES items like CNAME is a powerful combination to enable rapid
   synchronization in all cases.

   The "Rapid Synchronisation of RTP Flows" specification [<a href="./rfc6051" title="&quot;Rapid Synchronisation of RTP Flows&quot;">RFC6051</a>] does
   provide an analysis of the initial synchronization delay for
   different sessions depending on the number of receivers as well as on
   session bandwidth (<a href="./rfc6051#section-2.1">Section&nbsp;2.1 of [RFC6051]</a>).  These results are also



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<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


   applicable for other SDES items that have a similar time dependency
   until the information can be sent using RTCP.  These figures can be
   used to determine the benefit of reducing the initial delay before
   information is available for some use cases.

   [<a id="ref-RFC6051">RFC6051</a>] also discusses the case of late joiners and defines an RTCP
   Feedback format to request synchronization information, which is
   another potential use case for SDES items in the RTP header
   extension.  It would, for example, be natural to include a CNAME SDES
   item with the header extension containing the NTP-formatted reference
   clock to ensure synchronization.

   The ongoing work on bundling Session Description Protocol (SDP) media
   descriptions [<a href="#ref-SDP-BUNDLE">SDP-BUNDLE</a>] has identified a new SDES item that can
   benefit from timely delivery.  A corresponding RTP SDES compact
   header extension is therefore also defined and registered in that
   document:

   MID:  This is a media description identifier that matches the value
      of the SDP [<a href="./rfc4566" title="&quot;SDP: Session Description Protocol&quot;">RFC4566</a>] a=mid attribute [<a href="./rfc5888" title="&quot;The Session Description Protocol (SDP) Grouping Framework&quot;">RFC5888</a>], to associate RTP
      streams multiplexed on the same transport with their respective
      SDP media description.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Specification</span>

   This section first specifies the SDES item RTP header extension
   format, followed by some usage considerations.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  SDES Item Header Extension</span>

   An RTP header extension scheme allowing for multiple extensions is
   defined in "A General Mechanism for RTP Header Extensions" [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>].
   That specification defines both short and long item headers.  The
   short headers (one byte) are restricted to 1 to 16 bytes of data,
   while the long format (two bytes) supports a data length of 0 to 255
   bytes.  Thus, the RTP header extension formats are capable of
   supporting any SDES item from a data length perspective.

   The ID field, independent of a short or long format, identifies both
   the type of RTP header extension and, in the case of the SDES item
   header extension, the type of SDES item.  The mapping is done in
   signaling by identifying the header extension and SDES item type
   using a URN, which is defined in <a href="#section-5">Section 5</a> ("IANA Considerations")
   for the known SDES items appropriate to use.







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<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  One-Byte Format</span>

   The one-byte header format for an SDES item extension element
   consists of the one-byte header (defined in <a href="./rfc5285#section-4.2">Section&nbsp;4.2 of
   [RFC5285]</a>), which consists of a 4-bit ID followed by a 4-bit length
   field (len) that identifies the number of data bytes (len value +1)
   following the header.  The data part consists of len+1 bytes of UTF-8
   [<a href="./rfc3629" title="&quot;UTF-8, a transformation format of ISO 10646&quot;">RFC3629</a>] text.  The type of text and its mapping to the SDES item
   type are determined by the ID field value.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID   |  len  | SDES item text value ...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  Two-Byte Format</span>

   The two-byte header format for an SDES item extension element
   consists of the two-byte header (defined in <a href="./rfc5285#section-4.3">Section&nbsp;4.3 of
   [RFC5285]</a>), which consists of an 8-bit ID followed by an 8-bit length
   field (len) that identifies the number of data bytes following the
   header.  The data part consists of len bytes of UTF-8 text.  The type
   of text and its mapping to the SDES item type are determined by the
   ID field value.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ID       |      len      |  SDES item text value ...     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Usage of the SDES Item Header Extension</span>

   This section discusses various usage considerations: which form of
   the header extension to use, the packet expansion, and when to send
   SDES items in the header extension.

<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  One-Byte or Two-Byte Headers</span>

   The RTP header extensions for SDES items MAY use either the one-byte
   or two-byte header formats, depending on the text value size for the
   used SDES items and the requirement from any other header extensions
   used.  The one-byte header SHOULD be used when all non-SDES item



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   header extensions support the one-byte format and all SDES item text
   values contain at most 16 bytes.  Note that the RTP header extension
   specification [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>] does not allow mixing one-byte and two-byte
   headers for the same RTP stream (SSRC), so if any SDES item requires
   the two-byte header, then all other header extensions MUST also use
   the two-byte header format.

   For example, if using CNAMEs that are generated according to
   "Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names
   (CNAMEs)" [<a href="./rfc7022" title="&quot;Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)&quot;">RFC7022</a>], if using short-term persistent values, and if
   96-bit random values prior to base64 encoding are sufficient, then
   they will fit into the one-byte header format.

   An RTP middlebox needs to take care choosing between one-byte headers
   and two-byte headers when creating the first packets for an outgoing
   stream (SSRC) with header extensions.  First of all, it needs to
   consider all the header extensions that may potentially be used.
   Second, it needs to know the size of the SDES items that are going to
   be included and use two-byte headers if any are longer than 16 bytes.
   An RTP middlebox that forwards a stream, i.e., not mixing it or
   combining it with other streams, may be able to base its choice on
   the header size in incoming streams.  This is assuming that the
   middlebox does not modify the stream or add additional header
   extensions to the stream it sends, in which case it needs to make its
   own decision.

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  MTU and Packet Expansion</span>

   The RTP packet size will clearly increase when a header extension is
   included.  How much depends on the type of header extensions and
   their data content.  The SDES items can vary in size.  There are also
   some use cases that require transmitting multiple SDES items in the
   same packet to ensure that all relevant data reaches the receiver.
   An example of that is when CNAME, a MID, and the rapid time
   synchronization extension from <a href="./rfc6051">RFC 6051</a> are all needed.  Such a
   combination is quite likely to result in at least 16+3+8 bytes of
   data plus the headers, which will be another 7 bytes for one-byte
   headers, plus two bytes of header padding to make the complete header
   extension 32-bit word aligned, thus 36 bytes in total.

   If the packet expansion cannot be taken into account when producing
   the RTP payload, it can cause an issue.  An RTP payload that is
   created to meet a particular IP-level Maximum Transmission Unit
   (MTU), taking the addition of IP/UDP/RTP headers but not RTP header
   extensions into account, could exceed the MTU when the header
   extensions are present, thus resulting in IP fragmentation.  IP
   fragmentation is known to negatively impact the loss rate due to
   middleboxes unwilling or not capable of dealing with IP fragments, as



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   well as increasing the target surface for other types of packet
   losses.

   As this is a real issue, the media encoder and payload packetizer
   should be flexible and be capable of handling dynamically varying
   payload size restrictions to counter the packet expansion caused by
   header extensions.  If that is not possible, some reasonable worst-
   case packet expansion should be calculated and used to reduce the RTP
   payload size of all RTP packets the sender transmits.

<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>.  Transmission Considerations</span>

   The general recommendation is to only send header extensions when
   needed.  This is especially true for SDES items that can be sent in
   periodic repetitions of RTCP throughout the whole session.  Thus, the
   different usages (<a href="#section-4.2.4">Section 4.2.4</a>) have different recommendations.  The
   following are some general considerations for getting the header
   extensions delivered to the receiver:

   1.  The probability for packet loss and burst loss determine how many
       repetitions of the header extensions will be required to reach a
       targeted delivery probability and, if burst loss is likely, what
       distribution would be needed to avoid getting all repetitions of
       the header extensions lost in a single burst.

   2.  If a set of packets are all needed to enable decoding, there is
       commonly no reason for including the header extension in all of
       these packets, as they share fate.  Instead, at most one instance
       of the header extension per independently decodable set of media
       data would be a more efficient use of the bandwidth.

   3.  How early the SDES item information is needed, from the first
       received RTP data or only after some set of packets are received,
       can guide if the header extension(s) should be in all of the
       first N packets or be included only once per set of packets, for
       example, once per video frame.

   4.  The use of RTP-level robustness mechanisms, such as RTP
       retransmission [<a href="./rfc4588" title="&quot;RTP Retransmission Payload Format&quot;">RFC4588</a>] or forward error correction [<a href="./rfc5109" title="&quot;RTP Payload Format for Generic Forward Error Correction&quot;">RFC5109</a>],
       may treat packets differently from a robustness perspective, and
       SDES header extensions should be added to packets that get a
       treatment corresponding to the relative importance of receiving
       the information.

   As a summary, the number of header extension transmissions should be
   tailored to a desired probability of delivery, taking the receiver
   population size into account.  For the very basic case, N repetitions
   of the header extensions should be sufficient but may not be optimal.



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   N is selected so that the header extension target delivery
   probability reaches 1-P^N, where P is the probability of packet loss.
   For point-to-point or small receiver populations, it might also be
   possible to use feedback, such as RTCP, to determine when the
   information in the header extensions has reached all receivers and to
   stop further repetitions.  Feedback that can be used includes the
   RTCP Extended Report (XR) Loss RLE Report Block [<a href="./rfc3611" title="&quot;RTP Control Protocol Extended Reports (RTCP XR)&quot;">RFC3611</a>], which
   indicates successful delivery of particular packets.  If the RTP/AVPF
   transport-layer feedback message for generic NACK [<a href="./rfc4585" title="&quot;Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)&quot;">RFC4585</a>] is used,
   it can indicate the failure to deliver an RTP packet with the header
   extension, thus indicating the need for further repetitions.  The
   normal RTCP report blocks can also provide an indicator of successful
   delivery, if no losses are indicated for a reporting interval
   covering the RTP packets with the header extension.  Note that loss
   of an RTCP packet reporting on an interval where RTP header extension
   packets were sent does not necessarily mean that the RTP header
   extension packets themselves were lost.

<span class="h4"><a class="selflink" id="section-4.2.4" href="#section-4.2.4">4.2.4</a>.  Different Usages</span>

<span class="h5"><a class="selflink" id="section-4.2.4.1" href="#section-4.2.4.1">4.2.4.1</a>.  New SSRC</span>

   A new SSRC joins an RTP session.  As this SSRC is completely new for
   everyone, the goal is to ensure, with high probability, that all RTP
   session participants receive the information in the header extension.
   Thus, header extension transmission strategies that allow some
   margins in the delivery probability should be considered.

<span class="h5"><a class="selflink" id="section-4.2.4.2" href="#section-4.2.4.2">4.2.4.2</a>.  Late Joiner</span>

   In a multi-party RTP session where one or a small number of receivers
   join a session where the majority of receivers already have all
   necessary information, the use of header extensions to deliver
   relevant information should be tailored to reach the new receivers.
   The trigger to send header extensions can, for example, be either
   RTCP from a new receiver(s) or an explicit request like the Rapid
   Resynchronization Request defined in [<a href="./rfc6051" title="&quot;Rapid Synchronisation of RTP Flows&quot;">RFC6051</a>].  In centralized
   topologies where an RTP middlebox is present, it can be responsible
   for transmitting the known information, possibly stored, to the new
   session participant only and not repeat it to all the session
   participants.

<span class="h5"><a class="selflink" id="section-4.2.4.3" href="#section-4.2.4.3">4.2.4.3</a>.  Information Change</span>

   If the SDES information is tightly coupled with the RTP data, and the
   SDES information needs to be updated, then the use of the RTP header
   extension is superior to RTCP.  Using the RTP header extension
   ensures that the information is updated on reception of the related



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   RTP media, ensuring synchronization between the two.  Continued use
   of the old SDES information can lead to undesired effects in the
   application.  Thus, header extension transmission strategies with
   high probability of delivery should be chosen.

<span class="h4"><a class="selflink" id="section-4.2.5" href="#section-4.2.5">4.2.5</a>.  SDES Items in RTCP</span>

   The RTP header extension information, i.e., SDES items, can and will
   be sent also in RTCP.  Therefore, it is worth making some reflections
   on this interaction.  As an alternative to the header extension, it
   is possible to schedule a non-regular RTCP packet transmission
   containing important SDES items, if one uses an RTP-/AVPF-based RTP
   profile.  Depending on the mode in which one's RTCP feedback
   transmitter is working, extra RTCP packets may be sent as immediate
   or early packets, enabling more timely SDES information delivery.

   There are, however, two aspects that differ between using RTP header
   extensions and any non-regular transmission of RTCP packets.  First,
   as the RTCP packet is a separate packet, there is no direct relation
   and also no fate sharing between the relevant media data and the SDES
   information.  The order of arrival for the packets will matter.  With
   a header extension, the SDES items can be ensured to arrive if the
   media data to play out arrives.  Second, it is difficult to determine
   if an RTCP packet is actually delivered, as the RTCP packets lack
   both a sequence number and a mechanism providing feedback on the RTCP
   packets themselves.

<span class="h4"><a class="selflink" id="section-4.2.6" href="#section-4.2.6">4.2.6</a>.  Update Flaps</span>

   The SDES item may arrive both in RTCP and in RTP header extensions,
   potentially causing the value to flap back and forth at the time of
   updating.  There are at least two reasons for these flaps.  The first
   one is packet reordering, where a pre-update RTP or RTCP packet with
   an SDES item is delivered to the receiver after the first RTP/RTCP
   packet with the updated value.  The second reason is the different
   code paths for RTP and RTCP in implementations.  An update to the
   sender's SDES item parameter can take a different time to propagate
   to the receiver than the corresponding media data.  For example, an
   RTCP packet with the SDES item included that may have been generated
   prior to the update can still reside in a buffer and be sent
   unmodified.  The update of the item's value can, at the same time,
   cause RTP packets to be sent including the header extension, prior to
   the RTCP packet being sent.

   However, most of these issues can be avoided by the receiver
   performing some checks before updating the receiver's stored value.
   To handle flaps caused by reordering, SDES items received in RTP
   packets with the same or a lower extended sequence number than the



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   last change MUST NOT be applied, i.e., discard items that can be
   determined to be older than the current one.  For compound RTCP
   packets, which will contain an SR packet (assuming an active RTP
   sender), the receiver can use the RTCP SR timestamp field to
   determine at what approximate time it was transmitted.  If the
   timestamp is earlier than the last received RTP packet with a header
   extension carrying an SDES item, and especially if carrying a
   previously used value, the SDES item in the RTCP SDES packet can be
   ignored.  Note that media processing and transmission pacing can
   easily cause the RTP header timestamp field as well as the RTCP SR
   timestamp field to not match with the actual transmission time.

<span class="h4"><a class="selflink" id="section-4.2.7" href="#section-4.2.7">4.2.7</a>.  RTP Header Compression</span>

   When Robust Header Compression (ROHC) [<a href="./rfc5225" title="&quot;RObust Header Compression Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-Lite&quot;">RFC5225</a>] is used with RTP, the
   RTP header extension [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>] data itself is not part of what is
   being compressed and thus does not impact header compression
   performance.  The extension indicator (X) bit in the RTP header is,
   however, compressed.  It is classified as rarely changing, which may
   no longer be true for all RTP header extension usage, in turn leading
   to lower header compression efficiency.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  IANA Considerations</span>

   This section details the following updates made by IANA:

   o  Creation of a new sub-registry reserved for RTCP SDES items with
      the URN subspace "urn:ietf:params:rtp-hdrext:sdes:" in the "RTP
      Compact Header Extensions" registry.

   o  Registration of the SDES items appropriate for use with the RTP
      header extension defined in this document.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Registration of an SDES Base URN</span>

   IANA has registered the following entry in the "RTP Compact Header
   Extensions" registry:

   Extension URI: urn:ietf:params:rtp-hdrext:sdes
   Description:   Reserved as base URN for RTCP SDES items that are also
                  defined as RTP compact header extensions.
   Contact:       Authors of <a href="./rfc7941">RFC 7941</a>
   Reference:     <a href="./rfc7941">RFC 7941</a>

   The reason to register a base URN for an SDES subspace is that the
   name represents an RTCP Source Description item, for which a
   specification is strongly recommended [<a href="./rfc3550" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">RFC3550</a>].




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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Creation of the "RTP SDES Compact Header Extensions" Sub-Registry</span>

   IANA has created a sub-registry to the "RTP Compact Header
   Extensions" registry, with the same basic requirements, structure,
   and layout as the "RTP Compact Header Extensions" registry.

   o  Registry name: RTP SDES Compact Header Extensions

   o  Specification: <a href="./rfc7941">RFC 7941</a>

   o  Information required: Same as for the "RTP Compact Header
      Extensions" registry [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>]

   o  Review process: Same as for the "RTP Compact Header Extensions"
      registry [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>], with the following requirements added to the
      Expert Review [<a href="./rfc5226" title="">RFC5226</a>]:

      1.  Any registration using an extension URI that starts with
          "urn:ietf:params:rtp-hdrext:sdes:" (<a href="#section-5.1">Section 5.1</a>) MUST also
          have a registered Source Description item in the "RTP SDES
          item types" registry.

      2.  Security and privacy considerations for the SDES item MUST be
          provided with the registration.

      3.  Information MUST be provided on why this SDES item requires
          timely delivery, motivating it to be transported in a header
          extension rather than as RTCP only.

   o  Size and format of entries: Same as for the "RTP Compact Header
      Extensions" registry [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>].

   o  Initial assignments: See <a href="#section-5.3">Section 5.3</a> of this document.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Registration of SDES Item</span>

   IANA has registered the following SDES item in the newly formed "RTP
   SDES Compact Header Extensions" registry:

   Extension URI: urn:ietf:params:rtp-hdrext:sdes:cname
   Description:   Source Description: Canonical End-Point Identifier
                  (SDES CNAME)
   Contact:       Authors of <a href="./rfc7941">RFC 7941</a>
   Reference:     <a href="./rfc7941">RFC 7941</a>







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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Security Considerations</span>

   Source Description items may contain data that are sensitive from a
   security perspective.  There are SDES items that are or may be
   sensitive from a user privacy perspective, like CNAME, NAME, EMAIL,
   PHONE, LOC, and H323-CADDR.  Some may contain sensitive information,
   like NOTE and PRIV, while others may be sensitive from profiling
   implementations for vulnerability or other reasons, like TOOL.  The
   CNAME sensitivity can vary depending on how it is generated and what
   persistence it has.  A short-term CNAME identifier generated using a
   random number generator [<a href="./rfc7022" title="&quot;Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)&quot;">RFC7022</a>] may have minimal security
   implications, while a CNAME of the form user@host has privacy
   concerns, and a CNAME generated from a Media Access Control (MAC)
   address has long-term tracking potentials.

   In RTP sessions where any type of confidentiality protection is
   enabled for RTCP, the SDES item header extensions MUST also be
   protected.  This implies that to provide confidentiality, users of
   the Secure Real-time Transport Protocol (SRTP) need to implement and
   use encrypted header extensions per [<a href="./rfc6904" title="&quot;Encryption of Header Extensions in the Secure Real-time Transport Protocol (SRTP)&quot;">RFC6904</a>].  SDES items carried as
   RTP header extensions MUST then use commensurate strength algorithms
   and SHOULD use the same cryptographic primitives (algorithms, modes)
   as applied to RTCP packets carrying corresponding SDES items.  If the
   security level is chosen to be different for an SDES item in RTCP and
   an RTP header extension, it is important to justify the exception and
   to consider the security properties as the worst in each aspect for
   the different configurations.  It is worth noting that the current
   SRTP [<a href="./rfc3711" title="&quot;The Secure Real-time Transport Protocol (SRTP)&quot;">RFC3711</a>] only provides protection for the next trusted RTP/RTCP
   hop, which is not necessarily end to end.

   The general RTP header extension mechanism [<a href="./rfc5285" title="&quot;A General Mechanism for RTP Header Extensions&quot;">RFC5285</a>] does not itself
   contain any functionality that is a significant risk for a
   denial-of-service attack, neither from processing nor from storage
   requirements.  The extension for SDES items defined in this document
   can potentially be a risk.  The risk depends on the received SDES
   item and its content.  If the SDES item causes the receiver to
   perform a large amount of processing, create significant storage
   structures, or emit network traffic, such a risk does exist.  The
   CNAME SDES item in the RTP header extension is only a minor risk, as
   reception of a CNAME item will create an association between the
   stream carrying the SDES item and other RTP streams with the same
   SDES item.  This usually results in time synchronizing the media
   streams; thus, some additional processing is performed.  However, the
   application's media quality is likely more affected by an erroneous
   or changing association and media synchronization than the
   application quality impact caused by the additional processing.





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   As the SDES items are used by the RTP-based application to establish
   relationships between RTP streams or between an RTP stream and
   information about the originating participant, there SHOULD be strong
   integrity protection and source authentication of the header
   extensions.  If not, an attacker can modify the SDES item value to
   create erroneous relationship bindings in the receiving application.
   For information regarding options for securing RTP, see [<a href="./rfc7201" title="&quot;Options for Securing RTP Sessions&quot;">RFC7201</a>].

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  References</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3550">RFC3550</a>]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, <a href="./rfc3550">RFC 3550</a>, DOI 10.17487/RFC3550,
              July 2003, &lt;<a href="http://www.rfc-editor.org/info/rfc3550">http://www.rfc-editor.org/info/rfc3550</a>&gt;.

   [<a id="ref-RFC5285">RFC5285</a>]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", <a href="./rfc5285">RFC 5285</a>, DOI 10.17487/RFC5285, July
              2008, &lt;<a href="http://www.rfc-editor.org/info/rfc5285">http://www.rfc-editor.org/info/rfc5285</a>&gt;.

   [<a id="ref-RFC6904">RFC6904</a>]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", <a href="./rfc6904">RFC 6904</a>,
              DOI 10.17487/RFC6904, April 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6904">http://www.rfc-editor.org/info/rfc6904</a>&gt;.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Informative References</span>

   [<a id="ref-RFC3611">RFC3611</a>]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              <a href="./rfc3611">RFC 3611</a>, DOI 10.17487/RFC3611, November 2003,
              &lt;<a href="http://www.rfc-editor.org/info/rfc3611">http://www.rfc-editor.org/info/rfc3611</a>&gt;.

   [<a id="ref-RFC3629">RFC3629</a>]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, <a href="./rfc3629">RFC 3629</a>, DOI 10.17487/RFC3629, November
              2003, &lt;<a href="http://www.rfc-editor.org/info/rfc3629">http://www.rfc-editor.org/info/rfc3629</a>&gt;.

   [<a id="ref-RFC3711">RFC3711</a>]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              <a href="./rfc3711">RFC 3711</a>, DOI 10.17487/RFC3711, March 2004,
              &lt;<a href="http://www.rfc-editor.org/info/rfc3711">http://www.rfc-editor.org/info/rfc3711</a>&gt;.





<span class="grey">Westerlund, et al.           Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


   [<a id="ref-RFC4566">RFC4566</a>]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", <a href="./rfc4566">RFC 4566</a>, DOI 10.17487/RFC4566,
              July 2006, &lt;<a href="http://www.rfc-editor.org/info/rfc4566">http://www.rfc-editor.org/info/rfc4566</a>&gt;.

   [<a id="ref-RFC4585">RFC4585</a>]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", <a href="./rfc4585">RFC 4585</a>,
              DOI 10.17487/RFC4585, July 2006,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4585">http://www.rfc-editor.org/info/rfc4585</a>&gt;.

   [<a id="ref-RFC4588">RFC4588</a>]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", <a href="./rfc4588">RFC 4588</a>,
              DOI 10.17487/RFC4588, July 2006,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4588">http://www.rfc-editor.org/info/rfc4588</a>&gt;.

   [<a id="ref-RFC5109">RFC5109</a>]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", <a href="./rfc5109">RFC 5109</a>, DOI 10.17487/RFC5109, December
              2007, &lt;<a href="http://www.rfc-editor.org/info/rfc5109">http://www.rfc-editor.org/info/rfc5109</a>&gt;.

   [<a id="ref-RFC5225">RFC5225</a>]  Pelletier, G. and K. Sandlund, "RObust Header Compression
              Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
              UDP-Lite", <a href="./rfc5225">RFC 5225</a>, DOI 10.17487/RFC5225, April 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5225">http://www.rfc-editor.org/info/rfc5225</a>&gt;.

   [<a id="ref-RFC5226">RFC5226</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
              DOI 10.17487/RFC5226, May 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5226">http://www.rfc-editor.org/info/rfc5226</a>&gt;.

   [<a id="ref-RFC5576">RFC5576</a>]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", <a href="./rfc5576">RFC 5576</a>, DOI 10.17487/RFC5576, June 2009,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5576">http://www.rfc-editor.org/info/rfc5576</a>&gt;.

   [<a id="ref-RFC5888">RFC5888</a>]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", <a href="./rfc5888">RFC 5888</a>,
              DOI 10.17487/RFC5888, June 2010,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5888">http://www.rfc-editor.org/info/rfc5888</a>&gt;.

   [<a id="ref-RFC6051">RFC6051</a>]  Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", <a href="./rfc6051">RFC 6051</a>, DOI 10.17487/RFC6051, November 2010,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6051">http://www.rfc-editor.org/info/rfc6051</a>&gt;.

   [<a id="ref-RFC7022">RFC7022</a>]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)
              Canonical Names (CNAMEs)", <a href="./rfc7022">RFC 7022</a>, DOI 10.17487/RFC7022,
              September 2013, &lt;<a href="http://www.rfc-editor.org/info/rfc7022">http://www.rfc-editor.org/info/rfc7022</a>&gt;.




<span class="grey">Westerlund, et al.           Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


   [<a id="ref-RFC7201">RFC7201</a>]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", <a href="./rfc7201">RFC 7201</a>, DOI 10.17487/RFC7201, April 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7201">http://www.rfc-editor.org/info/rfc7201</a>&gt;.

   [<a id="ref-RFC7656">RFC7656</a>]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", <a href="./rfc7656">RFC 7656</a>,
              DOI 10.17487/RFC7656, November 2015,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7656">http://www.rfc-editor.org/info/rfc7656</a>&gt;.

   [<a id="ref-SDP-BUNDLE">SDP-BUNDLE</a>]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", Work in Progress,
              <a href="./draft-ietf-mmusic-sdp-bundle-negotiation-32">draft-ietf-mmusic-sdp-bundle-negotiation-32</a>, August 2016.




































<span class="grey">Westerlund, et al.           Standards Track                   [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc7941">RFC 7941</a>                  RTP HE for RTCP SDES               August 2016</span>


Acknowledgments

   The authors would like to thank the following individuals for
   feedback and suggestions: Colin Perkins, Ben Campbell, and Samuel
   Weiler.

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Farogatan 6
   SE-164 80 Stockholm
   Sweden

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com


   Bo Burman
   Ericsson
   Gronlandsgatan 31
   Stockholm  16480
   Sweden

   Email: bo.burman@ericsson.com


   Roni Even
   Huawei Technologies
   Tel Aviv
   Israel

   Email: roni.even@mail01.huawei.com


   Mo Zanaty
   Cisco Systems
   7100 Kit Creek
   RTP, NC  27709
   United States of America

   Email: mzanaty@cisco.com









Westerlund, et al.           Standards Track                   [Page 17]
</pre>

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